In a conventional ground improving method for forming a cylindrical ground improved portion, a pipe is drawn up while continuously being rotated. Also, in forming a vertical flat plate-shaped ground improved portion, the pipe is drawn up at a constant velocity without being rotated.
As is well known, in the case of a material like sediment that has a small uniaxial compressive strength and composes unbounded particles, there is an estrangement phenomenon of the grouting operation of the jet due to a grouting shear and a limitation in the grouting performance. Therefore, in the conventional technique, it is impossible to form an underground columnar consolidation body having a large section area or to improve a ground of a large area.
In the conventional technique, when considered from a microscale viewpoint, there are two kinds of portions, one of which is grouted by the jet and another of which is not fully grouted due to insufficient collisions of the jet against the ground. Therefore, there are large concave and convex portions on the surface of the grouted ground and it is impossible to execute a uniform grouting work. If the rotating speed and the drawing-up speed of the pipe are set to slow speeds, such concave and convex portions of the grouted ground can be slightly flattened. However, such slow rotating and drawing-up speeds remarkably deteriorate working efficiency.
Conventional techniques have been proposed in U.S. Pat. No. 4,084,648, entitled "PROCESS FOR THE HIGH-PRESSURE GROUTING WITHIN THE EARTH AND APPARATUS ADAPTED FOR CARRYING OUT SAME", and U.S. Pat. No. 4,047,580, entitled "HIGH-VELOCITY JET DIGGING METHOD".